Mobile high-frequency (or HF) 3-30 mHz antennas have, in general, been short for the frequency of operation. Because they are short, the antenna's loading coils are used to cancel out the capacitive reactance associated with short antennas, normally whip antennas. The loading coil can be placed at the base of a whip or can be put in the center of the whip, which is usually somewhere between 6 and 10 feet long. One can also put the loading coil at the top of the whip with a capacitive top load. While this top-loaded configuration works, the antenna can be made to operate effectively by bottom-loading the whip because it takes less inductance.
In the past, mobile antennas have been singled-banded, meaning they operate in one frequency range. These antennas can be made multi-banded by changing the frequency and tapping the coil used to load the antenna and shorting out the remainder of the coil. In a fairly recent innovation in the past 20 years, so-called screwdriver antennas have been developed, which are basically a center-loaded antenna having a variable turn coil. It will be noted that, in these configurations, the coil is fairly large in both length and diameter and usually has a cover that goes from the bottom of the coil to the top. The cover and internal shorting circuitry are motor-driven to move to short out portions of the coil as it is extended or contracted such that portions of the coil are shorted except the part of the coil that is used for the matching.
Typically, in such coils, the movable tap is driven by a DC motor with the motor being stopped at the point when the standing wave ratio (or SWR) is at a minimum. However, in order to change bands with such an antenna, the amount of time utilized in driving the motor is excessive such that to go from one band to another may take as many as 3 minutes. This is inconvenient when one wishes to shift from one band to another. It is likewise inconvenient when, within a band, one significantly tunes off the frequency at which the coil was originally set. Moreover, in the past for non-automatic screwdriver antennas, the coil is set by hand, which, for instance, requires the driver to get out of the car and move the tap.
In order to solve the inconvenience described above, there have been attempts to locate an antenna tuner at the base of the whip to effectuate impedance matching. However, antenna tuners are far less efficient than the use of a loading coil because of the stray capacitance at the output of the tuner. The capacitance and radiation resistance of the antenna is what is being fed by radio frequency (RF) energy. This stray capacitance is in parallel with the capacitance associated with the antenna itself. Thus, when one applies RF current to the antenna, the current is divided between the antenna and the stray capacitance. Note that the current created in the antenna causes radio waves to radiate. The more current that one can get into the antenna whip, the more it will radiate and the better it will perform. However, if more current is going into stray capacitance, then the amount of radiated power is diminished. While the tuner itself may include loading coils, it is nonetheless important to minimize stray capacitance by locating the loading coils on the antenna whip itself where the loading coil is not touching anything except the whip. This minimizes stray capacitance and provides a far better power transfer to the antenna. In antenna tuners, any loading coils are located within the antenna tuner itself.
Thus, the use of antenna tuners at the base of a whip has been largely rejected, and automatic screwdriver antennas have been substituted for the use of these antenna tuners. However, these automatic screwdriver antenna tuners are expensive and require either a manual or an expensive controller. Due to the external coil and the tapping arrangement, these antennas are big and heavy and are extremely costly. Moreover, they are unsightly if one is attempting to get a big efficient antenna. The small ones are better looking but do not work as well because of the Q factor of the coil. It is noted that one can hardly obtain an unloaded Q factor better than 500 to 600 out of any free-standing coil, and this requires relatively large size coils. Moreover, large coils with such a high Q factor limit the effective usable bandwidth of the antenna once it is tuned. Thus, there is a requirement for efficient mobile antennas to provide a high Q factor coil without being unsightly, large, and expensive.
There is, however, a base-loaded tunable mobile antenna produced by the Barrett Corporation of Australia, which utilizes a series of air-wound loading coils in a housing which are connected together to form the impedance matching function. The system requires a specialized transformer between the lower of the coils and the antenna feed point to transform the antenna impedance into one that matches the output of a transceiver, usually around 50 ohms. However, it is only with difficulty that these antennas can be made to match the transceiver output impedance. It is noted that, when the impedance matching offered exceeds a 2:1 SWR ratio, there is a folding back of the transmit power so that the antenna presents an SWR less than 2:1 SWR to the particular radio to which it is coupled. This requires specialized transformers that are designed for a particular transceiver. However, in terms of general-purpose amateur radios, absent a perfect match, these radios fold back the power so that these antennas do not always work particularly well.
Moreover, the Barrett antenna utilizes air-wound coils which, when placed in proximity to each other, crosstalk with each other such that the ability to effectuate a perfect match between the whip and the transceiver is impacted at various frequencies, making the matching unstable. In an effort to reduce crosstalk, the air-wound coils are oriented at right angles to each other. However, this technique only marginally reduces crosstalk.
Furthermore, if the relationship of the inductance values of each of the coils is not binary related, it makes switching schemes to switch these coils in and out an ad hoc process.
Finally, in the Barrett antennas, switching software is located at the base of the antenna where RF fields are high and oftentimes interfere with the semiconductor switching circuits located at the base of the whip. Housing the electronics for switching the coils of the Barrett antenna at the base of the whip, thus, presents instability problems, especially for the high currents involved when driving a whip-like antenna.
There is, therefore, a need for an automatic antenna tuning system for mobile whip antennas to eliminate the aforementioned problems.